Exhaust turbochargers have been widely known which improve an output of an engine used in automobiles and the like. More specifically, the exhaust turbocharger rotates a turbine with energy of exhaust gas from the engine, and compresses intake air with a centrifugal compressor directly connected to the turbine through a rotational shaft and supplies the resultant air into the engine.
A normal compressor in a performance comparison graph in FIG. 18 defined by a pressure ratio as a vertical axis and a flow rate as a horizontal axis represents the compressor (centrifugal compressor) of such an exhaust turbocharger. The compressor is stably operated in a flow rate range from a surge flow rate (a line on the left side in the diagram) at which surging as pulsation of the system as a whole occurs, and a choking flow rate (a line on the right side in the diagram) where choking occurs and the flow rate does not increase any further.
In a centrifugal compressor of a normal compressor type involving direct intake of air into the impeller, the flow rate range, between the coking flow rate and the surge flow rate, ensuring the stable operation is small. Thus, there is a problem in that the compressor needs to be operated at a low operation point which is far from the surge flow rate and thus leads to a low efficiency, to prevent the surging from occurring due to transient change during sudden acceleration.
To solve the problem, the following techniques have been developed. Specifically, guide vanes which generate a swirling flow of intake air are disposed on the upstream side of the impeller in the centrifugal compressor to increase the operation range of the exhaust turbocharger. Furthermore, the intake gas taken into the impeller is partially recirculated to the upstream side of the impeller in a housing of the supercharger to increase the operation range of the exhaust turbocharger.
The technique of providing a recirculation flow path to prevent the distal end side of the impeller leading edge from separating at the time of small flow rate operation involves a flow of air flowing over the impeller at a flow inlet of the recirculation flow path even at a maximum efficiency point at which the flow rate needs not to be improved, and thus the efficiency is degraded. As a result, the pressure ratio drops at a portion other than a low-flow-rate side (refer to the characteristics of a recirculation compressor in FIG. 18).
A technique, such as that in Patent Document 1 (Japanese Patent Application Laid-open No. 2005-23792) for example, has been further developed in which the operating range is increased by a combination of the recirculation flow path and the guide vanes, generating the swirling flow of the intake air, on the upstream side of the impeller.
The technique in Patent Document 1 is briefly described based on FIG. 19.
FIG. 19 shows the disclosed configuration including: guide vanes 03 disposed in an annular air chamber 01 disposed in a shroud portion; a circulation flow path 09 communicating with an intake communication path 05 open at a portion between a portion on the upstream side of the impeller and a portion in the vicinity of an impeller leading edge so that the compressed air can be introduced, and communicating with a discharge communication path 07 open on an intake port side on a portion on the upstream side of the impeller so that the compressed air can be discharged; and an air flow swirling mechanism 013 which is disposed at a portion of the flow path more on the upstream side than the discharge communication path 07 and swirls the air flow flowing into a rotating impeller 011 in the same direction as the impeller 011 and can adjust the swirling amount.